Hong Kong -LRB- CNN -RRB- -- At least 2,800 dead pigs have been fished from a Shanghai river since Friday , but authorities insist that tap water in the city is still safe to drink .

State news agency Xinhua said labels tagged to the pigs ' ears indicated they came from the upper waters of the Huangpu River , which flows through the center of Shanghai and is a source of the city 's drinking water .

It 's not clear why the pigs had been dumped in the river , though local media reported earlier this month that a disease had killed thousands of pigs in a village south of Shanghai .

`` We will continue to trace the source , investigate the cause , co-operate with neighboring areas and take measures to stop the dumping of pigs into rivers , '' the Shanghai Municipal Agricultural Commission said in a statement posted on their website on Monday .

As of Sunday , water quality on the Songjiang section of the river , where most of the pigs were found , remained normal and the incident has had `` no significant effect on tap water supply , '' the commission added .

However , local residents and users of of the popular Twitter-like microblog service Sina Weibo have expressed concern that the dead pigs would make the city 's tap water unsafe to drink .

`` Huangpu river is the source of drinking water for more than 20 million Shanghai residents . And this horrific incident was only made public when residents started posting pictures on Weibo , '' business investor Xue Manzi said in a post on his account .

The agricultural commission said it had tested organ samples from the pig carcasses and the results suggested the animals had contracted a type of porcine circovirus .

According to Professor Fred Leung , who specialises in animal diseases at Hong Kong University , this is a fairly common disease in pigs and not usually fatal on its own .

Pictures showed sanitation workers with sticks retrieving the bloated bodies of small pigs caught up in reeds and debris at the side of the river .

A local newspaper in Jiaxing , a city in Zhejiang province south of Shanghai , reported on March 6 that tens of thousands of pigs had died of an animal disease in a major pig farming village in the past two months .

`` According to our records , 10,078 pigs died in January , another 8,325 died in February . More than 300 pigs die everyday in our village , and we barely have any space left to dispose of the dead pigs , '' a local villager was quoted by the paper as saying .

Chen Yi , a veterinarian at the Shanghai Academy of Agricultural Sciences , told the Global Times newspapers that farmers are required by law to dispose of dead animals at community disposal sites or bury them with disinfectant .

CNN 's Zhang Dayu reported from Beijing

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NEW : Pig carcasses test positive for porcine circovirus

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More than 2,800 dead pigs found floating in Shanghai river

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Authorities say local tap water still safe to drink but residents worried

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Reports suggests that more than 10,000 pigs died in a neighboring province earlier this year